1. Field of the Invention
The present invention relates to a thermal transfer recording apparatus equipped with an ink sheet, for transferring wax or the like of the ink sheet onto paper.
2. Description of the Related Art
Printing apparatuses of the type that transfers wax or the like from an ink sheet onto paper by utilizing the heat of a thermal head are ubiquitously used as word processor printers, facsimile printing apparatuses, and the like. Of such printers, specifically, the type of printer that prints one line at a time widthwise on an A4, B4, or like-size sheet requires the use of a wide ink sheet which is prone to partially shrink due to heating during printing, thus tending to cause ink sheet transport failure and print failure. Various methods of correction using a sheet correcting plate, etc. have been tried to address the problem, but none of them have been successful in providing a perfect preventive measure.
Japanese Unexamined Patent Publication JP-A 4-148971 (1992) discloses a method in which ribbon guide rollers so arranged as to be able to align an ink ribbon (ink sheet) at the center are provided in the transport path of the ink ribbon to prevent wrinkling from occurring during recording and thereby to obtain high quality recording results. Further, Japanese Unexamined Patent Publication JP-A 4-25486 (1992) discloses a method in which an electrically conductive elastic member is provided in the transport path of the ink ribbon to ensure stable transport of the ink ribbon, thereby preventing image degradation from occurring due to wrinkling or slacking of the ink ribbon. On the other hand, Japanese Unexamined Patent Publication JP-A 7-266649 (1995) discloses a method in which the portion of a separation member which contacts the center portion of a thermal transfer ribbon is curved downward relative to both edges thereof, thereby absorbing relative stretching at the edges of the thermal transfer ribbon.
Printing techniques for transferring wax from an ink sheet onto paper by the heating of a thermal head are widely used. Specifically, when printing one line at a time along the width of a commonly used sheet such as an A4- or B4-size sheet, the width of the ink sheet is made equal to the length of the shorter side of the A4 or B4 size, for example.
During printing, the ink sheet partially shrinks due to the heat of the thermal head, as a result of which wrinkles are formed in the ink sheet and transport failure occurs, causing print failure. In particular, in the-case of a wide ink sheet, it is difficult to eliminate such print failure, and a measure that can perfectly prevent it has yet to be devised.
An object of the present invention is to prevent the causes of print failure, such as wrinkling and transport failure of the ink sheet, resulting from the partial shrinking of the ink sheet due to the heating of the thermal head during printing.
In a first aspect, the invention provides a thermal transfer recording apparatus comprising:
a thermal head (31); and
a platen roller (110) opposed to the thermal head (31),
the thermal transfer recording apparatus carrying out printing on a sheet (P) by causing a wax, a sublimation dye, or the like applied on an ink sheet (42) to melt or sublime by the heat of the thermal head (31), the ink sheet (42) being transported between the thermal head (31) and the platen roller (110),
wherein a tensile force applied to a portion of the ink sheet (42) which lies downstream of the thermal head (31) as viewed along a direction of ink sheet travel (d) is greater at both edges of the ink sheet (42) than at the center thereof.
According to the invention, the thermal transfer recording apparatus, comprising the thermal head and the platen roller opposed to the thermal head, prints on a sheet by causing a wax, a sublimation dye, or the like applied on the ink sheet to melt or sublime by the heat of the thermal head. In this thermal transfer recording apparatus, the take-up tensile force applied to the ink sheet is greater at both edges of the ink sheet than at the center thereof. With this arrangement, since vector components directed from the center toward both edges occur in the ink sheet tensile force, the ink sheet can be prevented from wrinkling due to the shrinking of the ink sheet caused by printing heat. Accordingly, the thermal transfer recording apparatus can prevent print failure and ink sheet transport failure.
In a second aspect of the invention, it is preferable that the thermal transfer recording apparatus further comprises a friction member for tensioning the ink sheet by applying friction to the ink sheet, wherein
the friction member is disposed downstream of the thermal head as viewed along the direction of travel,
a contact face of the friction member that is brought into contact with the ink sheet extends along full width of the ink sheet, and
a coefficient of friction at a center portion of the contact face of the friction member is higher than a coefficient of friction at both end portions of the contact face.
According to the invention, the friction member for frictionally tensioning the ink sheet is disposed downstream of the thermal head as viewed in the direction of ink sheet travel and in such a manner as to extend along the full width of the ink sheet. Further, the coefficient of friction of the contact face of the friction member is higher at the center portion than at both end portions thereof. The thus structured friction member can make the tensile force applied to both widthwise edges of the ink sheet greater than the tensile force applied to the center thereof. Accordingly, the thermal transfer recording apparatus can prevent print failure and ink sheet transport failure.
In a third aspect of the invention, it is preferable that a length of contact along which the center portion of the contact face of the friction member contacts the ink sheet is greater than a length of contact along which each of the end portions of the contact face of the friction member contacts the ink sheet.
According to the invention, the thermal transfer recording apparatus is constructed so that the length of contact along which the friction member for frictionally tensioning the ink sheet contacts the ink sheet is greater at the center than at both end portions. With this structure, the tensile force applied to both edges of the ink sheet can be made greater than that applied to the center thereof.
In a forth aspect of the invention, it is preferable that the length of contact along which each of the end portions of the contact face of the friction member contacts the ink sheet increases with increasing distance from each end of the contact face toward the center thereof.
According to the invention, the friction member can gradually reduce the ink sheet tensile force along the width thereof from both edges toward the center. Accordingly, the thermal transfer recording apparatus can prevent print failure and ink sheet transport failure more effectively.
In a fifth aspect of the invention, it is preferable that both end portions of the contact face of the friction member are each covered with a substance whose coefficient of friction is lower than the coefficient of friction of the center portion of the contact face.
According to the invention, in the thermal transfer recording apparatus, as a method of varying the coefficient of friction of the friction member for frictionally tensioning the ink sheet, a film such as a fluororesin sheet with a low coefficient of friction is glued to each end portion of the friction member, thus coating the end portion with fluororesin or the like. With this structure, the tensile force applied to center of the ink sheet can be made smaller than that applied to both edges thereof.
In a sixth aspect of the invention, it is preferable that the center portion of the contact face of the friction member is covered with a substance whose coefficient of friction is higher than the coefficient of friction of both end portions of the contact face.
In a seventh aspect of the invention, it is preferable that a film made of a substance whose coefficient of friction is higher than the coefficient of friction of both end portions of the contact face of the friction member is glued to the center portion of the contact face.
In an eighth aspect of the invention, it is preferable that the center portion of the contact face of the friction member is treated with surface roughening.
According to the sixth to eighth aspects of the invention, the center portion of the friction member for frictionally tensioning the ink sheet is coated with a substance or a film having a high coefficient of friction or is treated with surface roughening by sand blasting. Since this serves to increase the coefficient of friction of the center portion, the tensile force applied to the center portion of the ink sheet can be reduced compared to that applied to both end portions thereof.
As described above, according to the first to eighth aspects of the invention, by constructing the thermal transfer recording apparatus so that a greater take-up force is applied to both edges of the ink sheet than to the center thereof, vector components directed from the center toward both edges occur in the ink sheet pulling tensile force; this serves to prevent the ink sheet from wrinkling due to the shrinking of the ink sheet caused by printing heat. Accordingly, the thermal transfer recording apparatus can prevent print failure and ink sheet transport failure.